In closed circuit television (CCTV) systems used for surveillance or scene monitoring purposes, it is known to employ a plurality of television cameras, each of which is directed at a different scene. These scenes may be recorded making use of individual video recorders coupled to each camera output, the scenes then being available for review from the associated recorders. Such systems are generally unduly expensive for many installations.
In our prior U.S. Patent Application Ser. No. 099,148, filed Sept. 18, 1987, a system is described which is arranged to electronically encode identification information relative to each specific camera (video signal) source on the "back porch" of the vertical synchronizing portion of the associated video signal Thereafter, the video signals representing a predetermined plurality of image frames from each of the identified sources are recorded in a time-shared or sequential arrangement by a single video recorder on, for example, a single video tape.
The encoded identification information is used to recover and play back the multi-frame video signals from an individual, selected camera as recorded by the single recorder. Such arrangements are particularly cost effective both in terms of equipment usage and in use of recording media (tape).
In order to provide a continuous, stable display of the output previously recorded on the tape from a given camera, a storage type of display device such as a frame freeze monitor or the like is used. The frame freeze monitor is updated or refreshed whenever the next recording of the video signal from the selected camera (source) appears at the play back station of the video recorder If all of the video sources and the video recorder are synchronized to a common synchronizing signal source, the displayed image will be relatively stable and jitter-free over a period in which the monitor image is refreshed a number of times. Similarly, switching the monitor to display the output from different video sources will not result in an undesirable period of rolling or tearing of the image when a change occurs.
A particularly desirable operating mode for such systems is one in which only a single frame (two fields) or even a single field is recorded from each video source during a given surveillance cycle. In the case of single frame recording, two fields (a full image) are recorded from camera #1, the video recorder is then switched during a vertical synchronizing interval to the output from camera #2, the two field-one frame signal from camera #2 is recorded adjacent that from camera #1 on the video tape and so on. Such an arrangement conserves the recording tape and is suitable where all of the video sources and the recorder are operating synchronously (i.e., all are locked to the same synchronizing signals).
However, the foregoing desired results are not obtained in such a system without relatively closely locked and substantially similar video sources. If one or more of the video sources (cameras) is operating asynchronously, it may happen that the output from that source will result in recording of a portion of one image field, followed by a full field and then the remaining portion of another field during a time interval corresponding to the frame interval of the recorder. In another condition, even if each of two adjacent sets of recorded video information represent full frames from each of two sequential video sources, retrieval of two adjacent fields from the video recorder may, in fact, result in the combining of one field from one camera with one field from the next camera, thereby producing two superimposed images rather than a single interlaced image. Such a result is unacceptable.
In the foregoing type of arrangement (i.e. one video recorder and a plurality of video signal sources), improvements in versatility may be realized if the plurality of video signal sources and the video recorder can be operated without any common synchronization or locking together of the time base information utilized in the several devices.